Synergistic Enhancement of Birefringence in Metal Benzoxazolium Complexes by π-Conjugated [C ₉ H ₁₀ NO] ⁺ and Triangular [HgI ₃ ] ⁻ Motifs

Birefringent crystals can manipulate polarized light for lasers and precision instrumentation, and the excellent properties of typical ultraviolet birefringent crystals, e.g., BBO and CaCO 3, benefit from triangular planar π-conjugated groups such as (BO 3 ) 3– and (CO 3 ) 3– . In order to expand the applicable wavelength range of birefringent crystals to the mid-infrared region, d 10 configuration metal (Zn, Cd, Hg) iodides were combined with planar π-conjugated benzoxazolium (C 9 H 10 ON) + groups, and three new hybrid birefringent crystals, i.e., (ZnI 4 )(C 9 H 10 NO) 2 ( I ), (CdI 4 )(C 9 H 10 NO) 2 ( II ), and (HgI 3 )(C 9 H 10 NO) ( III ), were successfully synthesized by solution methods. Attributable to the synergistic enhancement of conjugated (C 9 H 10 ON) + and near-parallel alignment of the triangular [HgI 3 ] − units, compound III exhibits an obviously larger birefringence of 0.154 at 1060 nm than compounds I and II constructed from tetrahedrally coordinated [ZnI 4 ] 2– /[CdI 4 ] 2– . Further, the origin of anisotropic optical properties was elucidated via first-principles calculations. This study establishes that integrating the planar [HgI 3 ] − unit through an organic–inorganic hybrid approach can effectively enhance birefringence, providing a feasible design strategy for mid-infrared birefringent crystals.